SSI: fixed $date_local and $date_gmt without SSI (ticket #230).

SSI: fixed $date_local and $date_gmt without SSI (ticket #230).

If there is no SSI context in a given request at a given time,
the $date_local and $date_gmt variables used "%s" format, instead
of "%A, %d-%b-%Y %H:%M:%S %Z" documented as the default and used
if there is SSI module context and timefmt wasn't modified using
the "config" SSI command.

While use of these variables outside of the SSI evaluation isn't strictly
valid, previous behaviour is certainly inconsistent, hence the fix.